A towers or a masts may be constructed in many different ways according to its function and the requirements to the tower, such as size requirements, the load carrying capacity of the tower, the types of loads it needs to withstand, transportation issues, and the possibility to place equipment in the tower etc.
A traditional tower for a wind turbine is a tubular steel tower with tower section placed on top of each other fixed to a concrete foundation. Due to transport and production restrictions, the external diameter has a limited size. Therefore, often the shell thickness is seen as the primary dimensioning parameter when increasing the tower height.
Simply increasing the tower shell thickness is however a very ineffective way of achieving increased bearing capacity and stiffness as these parameters only grow linear with the shell thickness. In comparison, increasing the diameter of the tower yields an increased bearing capacity with the diameter in the power of two, and a stiffness growing with the diameter in three.
To overcome such size limitation problem some towers comprises a number of tower sections divided by vertical joints if the diameter is optimized or further tower sections placed on top of each other if the shell thickness in increased and thereby exceed the tower section weight limitation. Other towers have increased bearing capacity by having a lower part constructed of in situ concrete or concrete elements.
Alternatively or additionally, the bearing capacity of a tower such as a wind turbine tower can be increased and the stresses reduced in parts of the tower by installing a number of cables or wires from a number of anchors or foundations to attachment points on the tower. The cables provide stability to the tower to reduce oscillations from wind and reduces the loads in the part of the tower below the cables considerably. This so-called guyed or tethered tower will to some extent be de-loaded by the wire reactions and can thereby potentially be constructed by fewer long sections with a relatively smaller and thereby more transportable diameters.
However, the tethered tower inevitably takes up more land which has to be taken into account in planning the appliance of the tower and especially during erection and maintenance of the equipment on or in the tower, where the cables needs to be taken into account and may be very much in the way. Also, for a wind turbine, the tensioning and the attachments of the cables are crucial for the wind turbine tower to withstand the varying and potentially high wind forces which in the interacting with the rotor creates complex and at times quite extreme dynamic loads.
Further, the erecting of tethered towers is often a time consuming and costly process due to the positioning and tensioning of the cables which often requires extensive use of cranes and may be complicated by the surroundings of the tower such as vegetation, uneven ground etc.
As the towers become larger and larger as is seen for example with wind turbine towers, the erecting of the tower becomes correspondingly more evolved and complex requiring larger cranes to position the tower and to attach the cables. As the cables become longer, more than one crane or repositioning of the crane several times during the erecting process may be needed to position and attach all the cables.